Problems with fogging of silver halide emulsions have plagued the photographic industry from its inception. Fog can be defined as a developed density that is not associated with the action of the image-forming exposure, and is usually expressed as "D-min," the density obtained in the unexposed portions of the emulsion. Density, as normally measured, includes both that produced by fog and that produced as a function of exposure to light. It is known in the art that the appearance of photographic fog related to intentional or unintentional reduction of silver ion can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, spectral/chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions.
The suppression of fog is, thus, a major concern when dealing with silver halide emulsions. A multitude of compounds involving many different chemical structures have been studied and used for this purpose. Examples can be found in Research Disclosure 308119, published December 1989, and include mercaptotetrazoles, benzothiazoles, tetraazaindenes, disulfide compounds, and mercunic chloride to name a few. Despite the large number of materials available, few are able to reduce fog without also reducing speed.
There is an especially great need to control the tendency of emulsions to increase in fog while in the melted state. The demands of mass producing photographic film often require the pre-melting of large quantities of emulsion in preparation for long coating events. Emulsions may be held in the melted state for as long as 16 hours before completion of a particular job. During this period certain emulsions have a propensity to gain fog beyond an acceptable level and, therefore, require the addition of one or more antifoggants.
In this invention it has been discovered that a specific group of isothiazolin-ones is particularly useful in controlling fog which develops during melt-hold. Isothiazolin-ones are known as useful biocides for silver halide photographic elements as described in Research Disclosure, 37026, February 1995; in U.S. Pat. Nos. 4,224,403,and 4,490,462; and in JP 09-329862 and JP 10-011739. JP 09-133977 describes one class of isothiazolin-ones as reducing fog when added during precipitation of a silver halide emulsion. However, nowhere has it been recognized or desciubed that a specific group of isothiazolin-ones is useful in reducing the fog which may develop during melt-hold of certain silver halide emulsions.